x86/kexec: Clean up and document register use in relocate_kernel_64.S

Add more comments explaining what each register contains, and save the
preserve_context flag to a non-clobbered register sooner, to keep things
simpler.

No change in behavior intended.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Kai Huang <kai.huang@intel.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20241205153343.3275139-3-dwmw2@infradead.org

arch/x86/kernel/relocate_kernel_64.S

@@ -100,6 +100,9 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	movq	%r10, CP_PA_SWAP_PAGE(%r11)
 	movq	%rdi, CP_PA_BACKUP_PAGES_MAP(%r11)
 
+	/* Save the preserve_context to %r11 as swap_pages clobbers %rcx. */
+	movq	%rcx, %r11
+
 	/* Switch to the identity mapped page tables */
 	movq	%r9, %cr3
 
@@ -116,6 +119,14 @@ SYM_CODE_END(relocate_kernel)
 
 SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	UNWIND_HINT_END_OF_STACK
+	/*
+	 * %rdi	indirection page
+	 * %rdx start address
+	 * %r11 preserve_context
+	 * %r12 host_mem_enc_active
+	 * %r13 original CR4 when relocate_kernel() was invoked
+	 */
+
 	/* set return address to 0 if not preserving context */
 	pushq	$0
 	/* store the start address on the stack */
@@ -170,8 +181,6 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	wbinvd
 .Lsme_off:
 
-	/* Save the preserve_context to %r11 as swap_pages clobbers %rcx. */
-	movq	%rcx, %r11
 	call	swap_pages
 
 	/*
@@ -183,13 +192,14 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	movq	%cr3, %rax
 	movq	%rax, %cr3
 
+	testq	%r11, %r11	/* preserve_context */
+	jnz .Lrelocate
+
 	/*
 	 * set all of the registers to known values
 	 * leave %rsp alone
 	 */
 
-	testq	%r11, %r11
-	jnz .Lrelocate
 	xorl	%eax, %eax
 	xorl	%ebx, %ebx
 	xorl    %ecx, %ecx